There are several elements to securing a Linux server – however – top on the list should be assembling and implementing a firewall. Fortunately Linux comes bearing the gift of a free and robust firewall in Netfilter/iptables.
My latest column runs through the essentials of configuring a basic firewall to allow for the exploration of how iptables can cater to the web administrator. As with any exercise – this how-to is not all inclusive and consideration should be given to any specific needs and security issues one might have on their own systems and networks.
That said – the firewall built in this article should serve as a good starting point for controlling access to your servers.